They would file for a visitors visa, you can provide an I-134 to show that they will be supported while visiting, as well as a letter of invitation.
http://travel.state.gov/visa/temp/types/types_1262.htmlhttp://hanoi.usembassy.gov/visa_procedure.htmlThey need to demonstrate ties to the home country.
Example from consulate in Canada but this applies to any country:
QUOTE
In order to qualify for most categories of U.S. non-immigrant visas, you must be able to demonstrate to a U.S. Consular Officer that you have a permanent residence outside the United States that you do not intend to abandon. You may satisfy this requirement by showing you have strong economic, familial and social ties to Canada. "Ties" are factors that would require you to return to Canada upon completion of your temporary visit to the United States. Landed Immigrants in Canada must show ties to Canada.
Many applicants ask what documents they can bring to their visa interview that would demonstrate sufficiently their "ties" to their home country. The answer to this question is that no single document will be relevant to every case. Applicants should ask themselves the following question: "If I were visiting in the United States, what factors would cause me to end my visit and depart the United States?" Relevant factors, or "ties," outside the U.S. could be such things as a job, ownership of property, dependent children, or other important responsibilities that require your presence in your home country. You must demonstrate to the Consular Officer that these "ties" are strong enough to compel you to return to your home country upon completion of your visit to the United States.
http://montreal.usconsulate.gov/content/co...rting_documents